1. Field of the Invention
The present invention relates to an apparatus for determining an aperture value and, more specifically, to an apparatus for determining the aperture value in taking photographs with flash light.
The present invention further relates to a camera, especially a camera having automatic flash emitting function, in which flash light is automatically emitted when brightness of a subject is low.
2. Description of the Related Art
A lower limit of an aperture value is set in an adjustable range of the aperture value of a taking lens in order to improve photographing characteristics in taking photographs with flash light. For example, in Japanese Patent Laying-Open No. 35328/1987, the lower limit of the aperture value is determined such that even if a lens having a diameter for example (AV=1), is used, the aperture is stopped down smaller than a predetermined diameter, for example, F2.8 (AV=3).
By determining the lower limit of the aperture value as described above, the photographing characteristics in taking photographs with flash light can be improved. However, the object distance in which the brightness can be adjusted by the flash light becomes short, since the diameter of the taking lens is not large enough.
The above described problem will be described in detail in the following. As is well known, the following relation can be applied in which the guide number of the flash apparatus is represented by G, the F number of the aperture is represented by F.sub.no., and the object distance providing proper exposure when the flash light is fully emitted from the flash apparatus (that is, the longest object distance in which brightness can be adjusted by the flash light) is represented by D. EQU G=F.sub.no. .times.D
Therefore, if the lower limit in the aperture adjusting range is represented by F.sub.low in terms of the F number in taking photographs with flash light, the brightness can be properly adjusted by the flash light only in the range of D.ltoreq.G/.sub.Flow. For example, if the lower limit in the aperture adjustable range is F2.8 and the guide number G of the flash apparatus is 28, the brightness of the object can be properly adjusted by the flash light only when the object distance is shorter than or equal to 10 m. If the object is out of this range, the brightness can not be properly adjusted by the flash light.
Various cameras having low brightness-automatic flash emitting function have been known, in which flash photographing mode is automatically selected when the object is dark. In such cameras, whether the photographs should be taken with flash light or with ambient light is determined in accordance with either one of the following conditions.
(a) The shutter speed is slower than a limit for preventing blurring caused by camera shake, corresponding to the focal length of the taking lens,
(b) Brightness of the object is darker than a prescribed brightness value.
Table 3 shows values for controlling exposure near the point at which whether the photographing operation is to be carried out with flash light or not is switched in cameras employing the above conditions (a) and (b), respectively, with the flash synchronized shutter speed being TVx=6 (1/60 sec.), as an example.
In the Table, the reference character 35/F4 represents a taking lens whose focal length fl is 35 (mm) and whose full open aperture value AVo is 4 (F4). In the similar manner, the reference character 80/F5.6 represents a taking lens with fl=80 and AVo=5, and the reference character 200/F5.6 represents a taking lens with fl=200 and AVo=5. The reference character TVh represents the slowest shutter speed preventing blurring derived from camera shake The shutter speed TVh is dependent on the focal length fl of the taking lens and it can be represented as EQU TVh=log.sub.2 fl
For example, if the focal length fl=35, then TVh=log.sub.2 35.apprxeq.5.1. The reference characters IV represent maximum intensity (guide number) of the flash light. The maximum intensity (guide number) IV is dependent on the film sensitivity SV of the film and the range of illumination. Since the range of the illumination changes corresponding to the focal length fl, the maximum intensity IV changes corresponding to the film sensitivity SV and the focal length fl. In this example, the film sensitivity SV is selected to be 5 (ISO 100), and when fl .ltoreq.50, IV=2.6 (GNO.14) and when fl&gt;50, then IV=3.6 (GNO.20).
In the (a) type cameras, whether the flash light is to be emitted or not is determined in accordance with the shutter speed TVh preventing the blurring drive from camera shake, and the brightness value BV (switching point) at that time can be calculated by the following equation. EQU BV=TVh+AVo-SV
For example, when a lens of 35/F4 is used, ##EQU1## Similarly, when the lens of 80/F5.6 is used, the brightness value BV is 6.3, and when the lens of 200/F5.6 is used, the brightness value BV is 7.6. In the (b) type cameras, a reference brightness value BV (switching point) is set at 5, which is an intermediate value of the reference values BV in the (a) type cameras.
Based on an assumption that the distribution of brightness of the photographing field is uniform, the ratio of flash light to the ambient light is controlled to be 1:1 in taking photographs with flash light emission. More specifically, the shutter speed TV and the aperture value AV are determined such that the object is under exposed by IEV when the exposure is controlled only with the ambient light in taking photographs with flash light. In that case, the following relation can be applied with the brightness of the object represented by BV': EQU TV+AV-SV=BV'+1
Since the synchronized shutter speed TVx is set at 6 and the sensitivity of the film SV is set at 5 as mentioned above, the aperture value AV will be EQU AV=BV'
Near the switching point at which whether the flash light is to be emitted or not is switched, the brightness BV' of the object is equal to BVh in the (a) type cameras and the brightness BV' is 5 in the (b) type cameras Therefore, when a lens of 35/F4 is used, AV=4.1 and TV=6.0 in the (a) type cameras while AV=5.0 and TV=6.0 in the (b) type. The aperture value AV and the shutter speed TV can be calculated in the similar manner when the lens of 80/F5.6 and the lens of 200/F5.6 are used.
The longest object distance D(m) enabling proper photographing operation with flash light can be calculated based on the maximum intensity IV of the flash and the aperture value AV, in accordance with the following equation: ##EQU2## where .DELTA.EVfl is an amount of correction of the flash light, whose value is changed corresponding to the photographing magnification and to the ratio of the flash light to the ambient light. In this example, the ratio of the flash light to the ambient light is assumed to be 1:1 and the photographing magnification is assumed to be small, and therefore .DELTA.EVfl=-2. The amount of correction .DELTA.EVfl will be described in detail with reference to the embodiments later. When a lens of 30/F4 is used, the longest object distance D in which the brightness can be adjusted is 6.7(m) in the (a) type cameras, and the distance D is 4.9(m) in the (b) type cameras. The object distance D can be calculated in the similar manner when the lens of 80/F5.6 and of 200/F5.6 are used.
As is apparent from Table 3, in the (a) type cameras, the object distance D in which the brightness can be compensated by flash light, that is, in which proper photographing operation with flash light is possible, becomes very short as the focal distance fl of the taking lens becomes longer. For example, the distance D is 6.7 when the focal length fl of the lens is 35 but the distance D is only 2.8 when the focal length fl of the lens is 200. In the latter case, the proper exposure is hardly obtained.
Meanwhile, in the (b) type cameras, the point at which whether the flash light is to be emitted or not is determined is constant (in this example BV=5) regardless of the focal length fl of the lens. Therefore, in the example shown in the table, photographing operation is carried out with ambient light with TV=5 (1/30sec), even if the focal length fl of the lens is as long as 200, so that blurring derived from camera shake is very easy to occur. If the switching point is shifted to the higher brightness side in order to prevent such problem, the flash light may be emitted even though the photographing operation with ambient light is desired, when a lens having large diameter (small full open aperture value AVo) is used.
As described above, in the (a) type cameras, the object distance is very much limited in taking photographs with flash light when the focal length is long. In the (b) type cameras, there is a possibility of blurring derived from camera shake in taking photographs with ambient light when the focal length is long. And, if it is tried to prevent the possibility, then, when a lens having large diameter (small full open aperture value AVo) is used, the flash light may be emitted when photographing operation is to be carried out with ambient light.